Polyolefinic resins such as polypropylene have widely been used as materials for automobile parts because of excellent properties and low price. Unlike synthetic resins having polarity such as polyurethane resin, polyamide resin, acrylic resin and polyester resin, polyolefinic resins have no polarity and also have crystallinity, and thus they are not suited for coating and bonding.
To solve such a problem, there has hitherto been made trials to improve adhesion by activating the surface of a material made of the polyolefinic resin due to a plasma treatment or a gas flaming treatment. However, this method has drawbacks such as complicated processes, high equipment cost, large time loss, and variation in surface treatment effect due to an influence of complicated shape of the material, pigments and additives.
As disclosed in Japanese Patent Application, Second Publication No. Hei 6-2771, for example, various methods using a primer composition employed in the case of coating an automobile bumper with polypropylene have been proposed as a coating method without using such a surface treatment (pretreatment).
As disclosed in Japanese Patent Application, Second Publication No. Hei 1-16414, the primer composition includes, for example, a composition containing, as a main component, a chlorinated polyolefinic resin obtained by modifying polyolefins with an unsaturated carboxylic acid and/or its acid anhydride. Also, in Japanese Patent Application, Second Publication No. Hei 3-60872, the effect suited for use as a binder deposited to both a polyolefin and the other polar resin is confirmed by graft copolymerization of a chlorinated polypropylene and a liquid rubber with a polymerizable unsaturated compound.
However, these chlorinated polyolefinic resins have poor compatibility with acrylic, polyester and polyurethane resins contained in a primer composition and a top coating composition, and thus it is difficult to form a uniform film. Also, in the case of a modified composition using a liquid rubber, the resulting coating film sometimes exhibits tack, and thus compatibility and solution stability cannot be sufficiently improved.
As disclosed in Japanese Patent Application, First Publication No. 2002-309161, it has been found that compatibility and solution stability are improved by an. acrylic-modified chlorinated polyolefin obtained by bonding an acid-modified chlorinated polyolefin with a hydroxyl group-containing (meth)acrylate ester via an ester bond, thereby to introduce a double bond into the acid-modified chlorinated polyolefin, and graft-copolymerizing a (meth)acrylic acid monomer.
However, a coating composition for polyolefin material containing a resin obtained by the method for preparing an acrylic-modified chlorinated polyolefin disclosed in Japanese Patent Application, First Publication No. 2002-309161 has a fatal defect of properties of a coating composition, that is, phase separation occurs during the storage of the coating composition because of poor stability of the composition itself. Furthermore, since all components of the composition cannot be uniformly dissolved and lumps remain, “seeding” occurs in a coating film made of the composition and the coating film has poor appearance.